1. Field of the Invention
The present invention relates to a tension measuring device associated with ship mooring systems, particularly to a tension measuring device mounted on a mooring line for securing a ship to a fixed object such as an anchor, a dock or other structures.
2. Description of the Related Art
Generally, a mooring line extends from a winch of a ship to a fixed object such as an anchor or a dock. In the course of mooring the ship, the mooring line is subject to tension that fluctuates according to the oscillating behavior of the ships and external force exerted by tides, current and winds. Under turbulent wind and tidal conditions, the tension in a line may surge to a degree that may rupture the line.
Once a line is parted, remaining lines are subject to greater tension force in order to compensate for the lost tension in the disconnected line, which may lead to continuous rupturing of other lines. Once all the lines are disconnected, the ship will drift away from its preferred location and cause significant injuries or property losses. To prevent the mooring lines from rupturing, the tension in the mooring lines must be monitored constantly. If tension in one or several mooring lines exceed a certain limit, crew or other personnel aboard the ship will take precautionary or remedial measures, such as adding more lines or cast off and leave the dock.
Several systems have been suggested to monitor the tension in mooring lines. For example, FIG. 1 illustrates U.S. Pat. No. 4,171,640 to van Mastrigt who employs three rollers 22, 24 and 26 with radius R engaging a rope or cable 18 with unstresssed radius r1. The angle between the two tangent (contact) points, P8 is α. The downward force exerted by the cable to the central pulley 24 is 2F sin       a    2    .This downward force may be measured by a load cell which provides an indication of the tension in the rope. The personnel aboard the ship may monitor the tension in the line by reading a meter associated with the load cell.
Such a device, however, may provide inaccurate reading when applied to soft synthetic lines where the cross section changes with level of tension. It is also inaccurate when the mooring line wears out or flattens from prolonged use. When a smaller cable passes through this same arrangement, the angle between the two contact points reduces to β, as illustrated in FIG. 2. The downward force at the central pulley is now 2F sin       β    2    .The difference is the error in the system.
FIG. 2 shows the thickness of the cable 18, originally P7P8, reduced to P5P6 where the last contact point has shifted to the right to P6 when the line's cross section is smaller. Before the wear, the line 18 has a center of force M1 from which tensile force F would extend along the cable 18 perpendicular to the line P7P8. Once the line flattens, the center of force would be M2 somewhere between the points P5 and P6, and the tensile force F would not be perpendicular to the line P7P8, but rather to the line P5P6.
Another device for measuring tension in a mooring line employs sensors attached to the frame of the winch. Such a device is more complicated, expensive and the readings are indirect.
Still another system for measuring the tension in the line involves measuring the natural frequencies of a vibration in the tensioned line. This system was based on the idea that frequencies tend to increase when the tension in the line was stronger. A continued measurement of the tension, however, was not practical in this system because vibrations need to be excited each time before the tension in the line can be measured. Another problem in this system was that the vibration in the line might be influenced by changes in the unit weight of the line, possibly caused by rain, snow or other matters that may accumulate on the line.